The Tetrix (perfect) circle

Today I, or better the plotter, draw his first "perfect" circles.
It is not really a perfect circle and it would never be one. One reason is the construction used so far. Right now I use screw rods, which are first unsupported and secondly not straight. To improve the resolution of the circle, I have to buy expensive hardware, like ball-screws, bearings and encoders.
I wrote the program with the sine & cosine description of a circle, and with a proportional speed controller between both axes.
Another problem is that the motors do not start simultaneously, this cause an edge in the circle.
The proportional controller isn't accurate at all; the issue is that the motor accelerates at the beginning much faster as at the end of his power-reach. It is very difficult to find a good power-speed curve.

Next picture shows the circle drawing program:




Next are both cases for the proportional controller :

Next are both cases for the motor controller:

And of course the movie:


Next step is to engrave my name into wood...;)
Movie coming soon!

Tetrix CNC mill

This is a tough project! The aim is to build a mill to engrave letters or little signs into wood. You can find a bunch of cnc machines out of Lego, but no one can cut wood. So here is another example for what you can't build with Lego, but definitely with Tetrix.

The biggest problem is to find solutions for moving the table, because Tetrix doesn't have linear gears or something similar. This problem cannot be resolved by using Tetrix parts. So I bought 3 threat rods and 6 rod couplers for 10 $. Because Tetrix has no encoders in its basic kid and the "High Technic Encoders" are too expensive, I figured out that the "legacy angle Lego sensors" would be the best solution. Those sensors are not the most precise ones, but it would be sufficient for this application.

The next problem was about how to realize the guiding of the table. I found some interesting relations between the different profiles. But you have to try out a lot of things to find an optimal solution, because the most ideas you have, fail often on the whole relations.

The next pics show you some impressions of the construction.

The two pictures below show the guidance of the y and x direction.

Here you can see how I used the rod couplers. To use them as motor-coupler I had to bore them on the motor side up and to make a locking device.

http://www.youtube.com/watch?v=NDFuWjHLxuA

The circle has already been drawed but not filmed because it worked only once ... :)
Wood cutting and circles are coming soon....

Ben's race car

The idea was born at lunch-time. Because I needed a ride to get some food, my colleagues were joking that I have to build a car to get there by my own. I took it for serious and constructed a car with a Tetrix kid and some NXT parts.
This is probably one of the only things you can't build with Lego.... :)

My Tetrix LIKE and DISLIKE list #1

LIKE:
-screws
-steel
-strength and toughness of the whole concept
-very mechanic ... :)



DISLIKE:
-the reach of the hidden screws,
(using a ball-key would help a lot)


-the keys for the small screw heads are often breaking
(lose their profile)

Here the key is stocked in the screws head.





-servo mounting is not convenient.
You can't reach the screws without dismounting the servo.
So if you want to dismount the mechanism (red screws)
You have to dismount all the green screws (5x) to reach the red screws.





-the plug of the servos works only one way
if you plug it the wrong way nothing happens, there must be a sign or another connector so that you only can plug the right way.

-the Lego to Tetrix mounting piece is in my opinion a design fail:

The only way you can fix a Lego brick is shown on the right side on the pic, but sometimes you need it in the opposite way, like it is shown on the left side. But unfortunately it doesn't fit in.

-sometimes I miss some holes...I often ask me why only every 4th big hole has 8 small holes around?

Terry's little helper Robo Tixi

The story:
Terry just had surgery and has difficulty moving around while he is recovering.

Needs of the robot:
-Follow a path (black line)
-Pick up and deliver a bottle or what ever Terry needs
-Reach Terry in a comfortable area while he is sitting (18" high)
-use only one Tetrix kid and one Mindstorms kit

Problems:
-DC motors are very strong and powerful but they have lots of friction. When you want to make a line follower you usually need slow moves, so that the common solution is to gear the wheels down.

But in my case I had a problem: the servos are very accurate but not very powerful, so if you want to lift a bottle in Terry's reach, you have to use 2 servos parallel or build a transmission. But in one kid are only 4 gears and only 2 servos (and one I need for the grapple mechanism) so I used the following configuration:
-2 wheels are coupled directly to the DC motors
-4 gears and one servo to build a strong lifter
-1 servo for the grapple mechanism

More problems:
The biggest issue was: how to reach the height with one servo?
The more you gear, the less effective angle you receive, in result you have to choose a longer lever which means bigger forces.
Which effective angle I mean? One servo can make 180 degrees, if you gear it 2:1 you receive an effective angle of 90 degrees, with a gear step 4:1 you receive an effective angle of 45 degrees.

After several tests and transmission configurations the lifter still has problems to lift a bottle (with the longest lever).
I wasn't satisfied at all and so I searched for other solutions.
2 possibilities to try:
-work with kinetic structures to increase the reach with a smaller lever
-increase the strength of the servos by changing the affect point of the forces

I went the way to help the servo to carry up the bottle with a counter weight (I took batteries and a lever). This solution was very effective.

The last problem was still existing: the friction of the motors.
While following the line, it could happen that the motors get a very low energy input (lower than the motors need to override the motor intern friction). In this case the only thing you can do is to kick the lazy robot's a... :)
No not at all the solutions are very simple: before every new power order, you have to give the motors a quick power boost.
This is the final robot:

Welcome to my blog

Hi, I'm a mechanical engineering student at a German university and right now I am working in the States (as internship) on Lego (and of course Tetrix) engineering projects.
Every one knows Lego but I guess not Tetrix.... so Tetrix is like Lego, only made out of steel.
I try to build robots with Tetrix and Lego (and if needed also with other hardwares ... :) ).
My main projects are the constructions of robots that you can't build with Lego. During the following weeks I'm going to post my issues and my realized projects on this blog.

I hope you will enjoy it ....
Have fun... :)